Field of the Invention
The disclosure generally relates to the production of olefins and recycling specific product streams for greater production and efficiency. More specifically, the disclosure relates to the increased production of ethylene and propylene from a combination of a methanol to propylene (MTP) plant and a steam cracker plant and selective streams for recycling.
Description of the Related Art
Since the early production of olefins based synthetic polymers in the 1950's, the demand has steadily increased. Olefins, including ethylene and propylene, are the raw material for the production of long-chain synthetic polymers that can be created when ethylene and/or propylene gases are polymerized under specific conditions. Historically, steam cracking has been used to break bonds in longer chain molecules using feedstock such as ethane, propane, butane, naphtha and others. In recent years there is a trend to use lighter feedstock, especially ethane, for steam cracking. This trend will result in a shift of the ethylene to propylene ratio towards ethylene and therefore, a driver for on-purpose production of propylene. The art has progressed to using catalytic reactors to produce more propylene than the steam cracking process. For example US Publ. No. 2010/0234655A describes a process known as methanol-to-propylene (MTP) that is able to produce selectively a high amount of propylene by converting oxygenates into hydrocarbons. This technology is a catalytic process to produce on-purpose propylene using natural gas, coal or biomass as feedstocks. These alternative feedstocks are first converted to synthesis gas which is cleaned and then converted to methanol. Methanol in turn is converted to DME (dimethyl ether) which is used to produce a propylene-rich mixture containing various hydrocarbons. These various hydrocarbons are conditioned and fractionated into propylene and other components including ethylene if desired, LPG, and gasoline. From the outset, the MTP process had been based on a tailor made ZSM-5 zeolite catalyst. Efforts then began to combine the two processes.
US Publication No. 2010/0206771 teaches a combination of an MTP process and a steam cracking process. This publication teaches: “For the production of hydrocarbons, in particular C2-C4 olefins, using a combined plant with a steam cracker and at least one reactor for converting an educt mixture which includes steam and at least one oxygenate the respective intermediate product streams of the steam cracker and of the reactor are at least partly combined. To increase the yield of valuable products, a shape-selective zeolite material is used as catalyst in the reactor for oxygenate conversion and at least a part of the product streams obtained downstream of the combined plant is recirculated to the steam cracker and/or the reactor.”
US Publication No. 2015/0158783 also teaches a combination. This publication teaches: “The present invention provides an integrated process for the preparation of olefins, which process comprises the steps of: (a) reacting an oxygenate and/or olefinic feed in a reactor to form an effluent which comprises olefins; (b) fractionating at least part of the effluent into two olefinic product fractions; (c) subjecting a hydrocarbon feedstock in a reactor to a steam cracking process to form an effluent which comprises olefins including butadiene; (d) combining at least part of the first olefinic product fraction as obtained in step (b) and at least part of the second effluent which comprises olefins as obtained in step (c) to form a combined olefinic product stream comprising at least ethylene, propylene and butadiene; and (e) separating at least part of the combined olefinic product stream as obtained in step (d) to form a fraction comprising ethylene and/or propylene and a fraction that comprises butadiene.”
However, these combinations do not maximize ethylene and propylene regarding the management of the recycling of the different streams produced during the reaction. Therefore, there remains a need to further improve the production of ethylene and propylene from selective recycling of streams in a combined system and process.